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Enhanced quantum sensing with room-temperature solid-state masers
- Quantum Sensing
Quantum sensing takes advantage of quantum systems to measure various physical properties with remarkable precision. A popular platform for highly sensitive quantum sensing is solid-state electron spin systems, such as nitrogen-vacancy defects found in diamond. However, these systems face limitations due to quantum mechanical uncertainty and suboptimal methods for extracting the information.
Researchers have explored the use of a device known as a room-temperature maser to improve the performance of solid-state ensemble spin sensors in quantum sensing applications. The maser contributes to sharpening the magnetic resonance line shape, which is crucial for sensing with high accuracy. This allows for sensitive magnetometry, the measurement of magnetic fields, near zero-field conditions. By using this approach, they achieved a sensitivity of $42 pT/√Hz$, a significant advancement compared to previous methods.
Masing-enhanced quantum sensing involves a group of solid-state spins that interact with microwave radiation in a particular way. This interaction leads to an increase in the number of microwave photons within a resonator. When the number of spins reaches a specific threshold, the system becomes more efficient, resulting in a substantially clearer signal.
The researchers studied the relationship between the number of spins and the number of detected microwave photons in the system. They found that when the number of spins exceeded a certain threshold, a small increase in the number of spins led to a significant increase in detected microwave photons. This result indicated that the system has a nonlinear response, which can be beneficial for improving sensitivity.
To further their investigation, the team experimented with a material called pentacene-doped p-terphenyl, which possesses the unique property of supporting masing at room temperature and zero magnetic field. They used a special microwave device, known as a frequency-tunable regenerative microwave oscillator, to study the magnetic resonance features of the sample under various conditions.
The study examined how masing affected the magnetic resonance line shape, which is crucial for achieving high sensitivity in quantum sensing applications. They found that masing significantly enhanced the resonance amplitude by a factor of $78$, confirming the potential of maser action for improving sensitivity. The asymmetric shape of the pentacene transition line is due to interactions between photoexcited triplet electrons and the $14$ protons of the pentacene molecule, known as second-order hyperfine interactions.
Researchers also explored the possibility of using a spin ensemble as an organic solid-state magnetometer for quantum sensing applications. This type of magnetometer detects microwave signals rather than optical ones, offering faster initialization times than other optical readout schemes. The masing process enhances readout efficiency and increases sensitivity, resulting in a high signal-to-noise ratio of $133$.
To further improve the performance of the magnetometer, the researchers suggested increasing the bias dc magnetic field and deuterating (replacing hydrogen atoms with deuterium) the pentacene to suppress unwanted interactions, known as hyperfine interactions. They also proposed careful tuning of the optical pump energy to create an optimal condition where only triplet subensembles near the resonance reach the masing threshold, leading to further magnetic resonance linewidth reduction.
Additionally, to achieve a broader dynamic range with a linear response, the magnetometer can be adjusted to operate above the masing threshold instead of near it. Modifying optical pump powers or tuning driving amplitudes can produce similar effects as adjusting optical pump powers. If the maser can operate continuously with a large bias magnetic field, the maser output frequency will change linearly with magnetic fields. This strategy, initially proposed based on continuous-wave room-temperature masers, predicts that such a magnetometer has a sensitivity of $1 \sim 10 pT/√Hz$.
The study confirms that utilizing maser action in solid-state ensemble spin sensors can enhance sensitivity and overcome certain limitations. Masing -enhanced readout can provide higher signal-to-noise ratios and improved sensitivity. This technique is compatible with various sensing applications, such as electric/magnetic field sensing and dark matter searches. It also has the potential to lead to the development of even more advanced quantum sensors.
Required Additional Study Materials
- C. L. Degen, F. Reinhard, P. Cappellaro, Quantum sensing. Rev. Mod. Phys. 89, 035002 (2017).
- G. Wolfowicz, F. J. Heremans, C. P. Anderson, S. Kanai, H. Seo, A. Gali, G. Galli, D. D. Awschalom, Quantum guidelines for solid-state spin defects. Nat. Rev. Mater. 6, 906–925 (2021). J. F. Barry, J. M. Schloss, E. Bauch, M. J. Turner, C. A. Hart, L. M. Pham, R. L. Walsworth, Sensitivity optimization for NV-diamond magnetometry. Rev. Mod. Phys. 92, 015004 (2020). L. C. Rondin, J.-P. Tetienne, T. Hingant, J.-F. Roch, P. Maletinsky, V. Jacques, Magnetometry with nitrogen-vacancy defects in diamond. Rep. Prog. Phys. 77, 056503 (2014). J. Köhler, Magnetic resonance of a single molecular spin. Phys. Rep. 310, 261–339 (1999).
Introductory material
- “Quantum Solid-State Physics” by Serghey V. Vonsovsky and Mikhail I. Katsnelson
- “Solid State Physics” by David W. Snoke
Reference
Wu, Hao. et al. Enhanced quantum sensing with room-temperature solid-state masers. Science Advances 8 (2022).
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